As is well known in the art, a planetary-gear train comprises a sun gear, a ring gear concentrically surrounding the sun gear, and one or more planet gears meshing with both the sun gear and the ring gear while being mounted on a planet carrier rotatable about the axis of the latter. A desired transmission ratio between an input shaft and an output shaft is obtainable by rigidly coupling these shafts with two of the aforementioned components (sun gear, ring gear and planet carrier) while immobilizing the third component. In the system here particularly considered, the ring gear is held substantially stationary under normal operating conditions while the other components are respectively connected with the two shafts.
When the ring gear is allowed a certain rotational mobility relative to a surrounding housing, as by being linked with that housing through a spring or similar yieldable retaining means, that gear will occupy a predetermined relative position on standstill and during idling but will progressively deviate from that position under increasing load. If the input shaft is connected with the sun gear, the direction of deviation under load will be opposite the sense of rotation of the codirectionally turning shafts; in the event of a negative load, as where the output shaft carries a flywheel and tends to outrun the input shaft during deceleration of the source of driving power, the deviation occurs in the sense of shaft rotation. If the input shaft is connected with the planet carrier, the relative sense of deviation is reversed.
It has already been proposed to provide such a ring gear, engaged by a restoring spring, with a spring-loaded control pin which is radially guided in the housing and is cammed outwardly by a ramp formation on the ring gear when the latter deviates in either direction from its normal position relative to the housing. The control pin then acts upon a switch which signals the existence of an overload by disconnecting the drive motor, braking the load or emitting an alarm indication. The overload signal is the same for either direction of displacement of the ring gear. In practice, however, the corrective measures to be taken may have to be different in the two situations referred to above, i.e. when the output shaft lags the input shaft under load or leads it because of the flywheel effect (with due consideration to the step-down or step-up ratio of the planetary-gear train).